The invention relates to a method of imaging the blood flow in a vascular tree of an object to be examined, as well as to an X-ray device for carrying out such a method.
EP 860 696 A2 describes the formation of a series of X-ray projection images, that is, from different projection directions, of an object to be examined whose vascular tree is filled with a contrast medium. A three-dimensional image that contains a representation in space of the vascular tree is derived from said (two-dimensional) X-ray projection images by means of a known reconstruction algorithm.
For various applications, such as the analysis of pathologies of the cerebral vascular tree, however, further information is required in addition to the image of the vascular tree.
Therefore, it is an object of the present invention to provide a method that offers additional information, and also to provide an X-ray device for carrying out such a method.
This object is achieved in accordance with the invention by means of a method of imaging the blood flow in a vascular tree of an object to be examined, which method includes the steps of:
a) forming a series of X-ray projection images of the object to be examined from different projection directions,
b) reconstructing a three-dimensional image that contains an image of the vascular tree from the X-ray projection images,
c) determining a series of clusters from spatially coherent voxels in the three-dimensional image, at least one cluster that is defined by the direction of the blood flow being determined as from a start cluster of the vascular tree, said cluster itself acting as the starting point for the determination of at least one next cluster in the sequence and at least some of its voxels adjoining the voxels of the start cluster and of the next cluster, and for each new cluster of the sequence there being determined the subsequent cluster.
Because the next clusters in the direction of blood flow in the vascular tree are determined as from a predetermined voxel or cluster (start cluster), additional information is obtained that offers an examiner an impression of the propagation of the contrast medium or the blood flow in the vascular tree. In another embodiment practicing aspects of the present invention, the sequence of clusters determined can be made recognizable in an image, for example, by reproducing the sequence of clusters in different colors or by highlighting, in a dynamic representation, the clusters of the vascular tree successively in conformity with the sequence of clusters determined.
Even though more thorough analysis of the vascular tree is thus possible, these steps alone does not suffice to derive information concerning the actual progression in time of the blood flow in the vascular tree. In the previous, non-prepublished German patent application 10000185.8 in the name of applicant (PHDE000001), therefore, it is proposed to derive information concerning the propagation of the contrast medium in the vascular tree from a series of X-ray projection images that represent the vascular tree in different phases of inflow of a contrast medium, and to determine the times of arrival of the so-called contrast medium bolus (being the foremost part of the contrast medium as seen in the direction of blood flow) in the various parts of the vascular tree.
The problems that may then occur, for example, due to inaccurate determination of the time of arrival or due to the fact that vessels overlap in the projection images, can be solved. For example, in another embodiment practicing an aspect of the present invention, the temporal information obtained is combined with the information concerning the clusters of the vascular tree. An additional advantage resides in the fact that the times of arrival are not determined for individual voxels, but for clusters that usually consist of a number of voxels. The noise in the X-ray projection images then has less effect on the determination of the times of arrival.
The temporal information obtained and the information concerning the clusters of the vascular tree can be combined in various ways. For example, a cluster cannot have a time of arrival that is earlier than a cluster that precedes the relevant cluster in the sequence, so that correction of the time of arrival determined for the contrast medium bolus is possible already by means of this plausibility criterion. Such inaccuracies, however, can also be corrected by filtering in conformity with another embodiment practicing an aspect of the present invention wherein, the times of arrival of the sequence of clusters determined are subjected to median filtering for the purpose of modification.
Yet another embodiment practicing an aspect of the present invention enables the correct time of arrival to be assigned to clusters that overlap in the projection image. For example, an overlapping projection of clusters in the X-ray projection images and multiple arrival of the contrast medium bolus in this projection, the times of arrival are assigned to the associated clusters in the three-dimensional image in dependence on the times of arrival determined each time for the respective neighboring clusters in the vascular tree.
When the times of arrival thus determined are differentiated in the direction of the center lines of the vessels of the vascular tree, speed information is obtained that can also be reproduced (for example in color) in the three-dimensional image of the vascular tree; such information enables additional interpretations, for example, in conjunction with the radius of the vessel. For example, a high speed and a small diameter of the vessel in a given cluster may be an indication of a stenosis.
It would be possible in principle to derive the information concerning the progression in time of the blood flow from the same series of X-ray projection images as that wherefrom the three-dimensional image of the vascular tree is reconstructed; this would have the advantage that the radiation dose for the patient is not increased. However, in that case the inflow of the contrast medium cannot be tracked with the necessary accuracy. This drawback can be avoided by means in which two series of X-ray projection images are formed for the extraction of the spatial information and of the temporal information.
In addition to the vascular tree other structures are also reproduced in the three-dimensional image that is derived from the series of X-ray projection images, for example, bones that could disturb the evaluation. This drawback can be avoided by means in which the vascular tree is segmented. In the simplest case this segmentation may consist in that all voxels in the three-dimensional image that are below a given value are assigned to the vascular tree and the reproduction of all other voxels is suppressed.
An X-ray device for carrying out aspects of the method in accordance with the invention comprises an imaging unit (1) with an X-ray source (12) and an X-ray detector (13) for forming a series of X-ray projection images (Di) of the object (3) to be examined from different projection directions, and
an arithmetic unit (19) for reconstructing a three-dimensional image that contains a rendition of the vascular tree from the X-ray projection images (Di) and for determining a sequence of clusters from spatially coherent voxels in the three-dimensional image, where at least one cluster that is defined by the direction of the blood flow is determined as from a start cluster of the vascular tree, said cluster itself acting as the starting point for the determination of at least one next cluster in the sequence and at least some of its voxels adjoining the voxels of the start cluster and of the next cluster, and for each new cluster of the sequence there being determined the subsequent cluster. When parts of the vascular tree extend in the projection direction during the acquisition of the series of X-ray projection images that represent the various phases of the inflow of a contrast medium, accurate analysis of such parts of the vascular tree is not possible. This drawback can be avoided when use is made of two X-ray systems with projection directions that are preferably 90xc2x0 offset relative to one another.
The following description, claims and accompanying drawings set forth certain illustrative embodiments applying various principles of the present invention. It is to be appreciated that different embodiments applying principles of the invention may take form in various components, steps and arrangements of components and steps. These described embodiments being indicative of but a few of the various ways in which some or all of the principles of the invention may be employed in a method or apparatus. The drawings are only for the purpose of illustrating an embodiment of an apparatus and method applying principles of the present invention and are not to be construed as limiting the present invention.